Java并發(fā)Lock接口實(shí)現(xiàn)示例詳解

更新時(shí)間：2023年06月22日 09:07:23 作者：右耳菌

這篇文章主要為大家介紹了Java并發(fā)Lock接口，有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進(jìn)步，早日升職加薪

Locks包類層次結(jié)構(gòu)

Lock接口

方法簽名	描述	說(shuō)明
void lock();	獲取鎖(不死不休)	一直獲取鎖，直到拿到為止
boolean tryLock();	獲取鎖(淺嘗輒止)	嘗試獲得鎖，獲取不到就算了
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;	獲取鎖(過(guò)時(shí)不候)	超時(shí)限制，超過(guò)時(shí)間就放棄
void lockInterruptibly() throws InterruptedException;	獲取鎖(任人擺布)	可以在外部通過(guò)方法中斷
void unlock();	釋放鎖
Condition newCondition();

結(jié)論:

1、lock()最常用;

2、lockInterruptibly()方法一般更昂貴，有的impl可能沒(méi)有實(shí)現(xiàn)lockInterruptibly()，只有真的需要效應(yīng)中斷時(shí)，才使用，使用之前看看impl對(duì)該方法的描述。

trylock

package lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class GetLock_Demo {
    static Lock lock = new ReentrantLock();
    public static void main(String[] args) throws InterruptedException {
        lock.lock(); //主線程拿到鎖
        Thread th = new Thread(new Runnable() {
            @Override
            public void run() {
                boolean rs = lock.tryLock();
                System.out.println("是否獲取到鎖: " + rs);
            }
        });
        th.start();
        Thread.sleep(2000L);
        th.interrupt();//中斷線程運(yùn)行
        System.out.println("th 線程被中斷了");
    }
}

是否獲取到鎖: false
th 線程被中斷了

trylock帶超時(shí)

package lock;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class GetLock_Demo {
    static Lock lock = new ReentrantLock();
    public static void main(String[] args) throws InterruptedException {
        lock.lock(); //主線程拿到鎖
        Thread th = new Thread(new Runnable() {
            @Override
            public void run() {
                boolean rs = false;
                try {
                    rs = lock.tryLock(1, TimeUnit.SECONDS);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("是否獲取到鎖: " + rs);
            }
        });
        th.start();
        Thread.sleep(2000L);
        th.interrupt();//中斷線程運(yùn)行
        System.out.println("th 線程被中斷了");
    }
}

是否獲取到鎖: false
th 線程被中斷了

lockInterruptibly

package lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class GetLock_Demo {
    static Lock lock = new ReentrantLock();
    public static void main(String[] args) throws InterruptedException {
        lock.lock(); //主線程拿到鎖
        Thread th = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lock.lockInterruptibly();
                } catch (InterruptedException e) {
                    System.out.println("獲取鎖時(shí)被中斷了");
                    e.printStackTrace();
                }
            }
        });
        th.start();
        Thread.sleep(2000L);
        th.interrupt();//中斷線程運(yùn)行
        System.out.println("th 線程被中斷了");
    }
}

th 線程被中斷了
獲取鎖時(shí)被中斷了
java.lang.InterruptedException
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:898)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
at lock.GetLock_Demo$1.run(GetLock_Demo.java:16)
at java.lang.Thread.run(Thread.java:748)

lock and unlock

package lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class GetLock_Demo {
    static Lock lock = new ReentrantLock();
    public static void main(String[] args) throws InterruptedException {
        lock.lock(); //主線程拿到鎖
        Thread th = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("嘗試獲得鎖");
                lock.lock();
                System.out.println("獲得鎖了");
            }
        });
        th.start();
        Thread.sleep(2000L);
        th.interrupt();//中斷線程運(yùn)行
        System.out.println("th 線程被中斷了");
        Thread.sleep(5000L);
        lock.unlock();
    }
}

嘗試獲得鎖
th 線程被中斷了
獲得鎖了

Condition

Condition 一般是將其中的await和signal成對(duì)使用的，且一般是await在前signal在后，而且調(diào)用的使用，應(yīng)該確保本身是獲取到鎖的情況下，不然會(huì)出現(xiàn)以下問(wèn)題：

1. await 和 signal 方法應(yīng)該在lock內(nèi)部調(diào)用，否則會(huì)發(fā)生 IllegalMonitorStateException 異常

package lock;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Condition_Demo {
    private static Lock lock = new ReentrantLock();
    private static Condition condition = lock.newCondition();
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread() {
            @Override
            public void run() {
                lock.lock();
                try {
                    System.out.println("當(dāng)前線程:" + Thread.currentThread().getName() + "獲得鎖");
                    condition.await(); //因?yàn)檫@里將線程掛起，所以后面無(wú)法執(zhí)行
                    System.out.println("當(dāng)前線程:" + Thread.currentThread().getName() + "開(kāi)始執(zhí)行~");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock();
                }
            }
        };
        thread.start();
        Thread.sleep(2000L);
        System.out.println("休眠2s，來(lái)控制線程");
        condition.signal(); //直接喚醒會(huì)報(bào)錯(cuò)，因?yàn)閘ock方法執(zhí)行在Thread-0線程內(nèi)部,而我們代碼在這里執(zhí)行的是main線程,所以會(huì)報(bào)錯(cuò),
    }
}

當(dāng)前線程:Thread-0獲得鎖
休眠2s，來(lái)控制線程
Exception in thread "main" java.lang.IllegalMonitorStateException
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.signal(AbstractQueuedSynchronizer.java:1939)
at lock.Condition_Demo.main(Condition_Demo.java:33)

2. signal應(yīng)該在await后調(diào)用，否則會(huì)導(dǎo)致死鎖

package lock;
import sync.ReentrantLockDemo;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Condition_Demo {
    private static Lock lock = new ReentrantLock();
    private static Condition condition = lock.newCondition();
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000L);
                    System.out.println("休眠3秒，等待主線程先執(zhí)行.");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                lock.lock();
                try {
                    System.out.println("當(dāng)前線程:" + Thread.currentThread().getName() + "獲得鎖");
                    condition.await(); //因?yàn)檫@里將線程掛起，所以后面無(wú)法執(zhí)行
                    System.out.println("當(dāng)前線程:" + Thread.currentThread().getName() + "開(kāi)始執(zhí)行~");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock();
                }
            }
        };
        thread.start();
        Thread.sleep(2000L);
        System.out.println("休眠2s，來(lái)控制線程");
        lock.lock();
        condition.signal(); //直接喚醒會(huì)報(bào)錯(cuò)，因?yàn)閘ock方法執(zhí)行在Thread-0線程內(nèi)部,而我們代碼在這里執(zhí)行的是main線程,所以會(huì)報(bào)錯(cuò),
        lock.unlock(); //獲取到了這把鎖，然后解鎖.
        //2.當(dāng)然這里會(huì)出現(xiàn)死鎖的，如果signal方法在我們的await之前執(zhí)行，那么這里就會(huì)死鎖
    }
}

休眠2s，來(lái)控制線程
休眠3秒，等待主線程先執(zhí)行.
當(dāng)前線程:Thread-0獲得鎖
// 這里死鎖了

使用condition實(shí)現(xiàn)阻塞隊(duì)列的例子

package lock;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class BlockingQueue_Demo {
    public static void main(String[] args) throws InterruptedException {
        BlockingQueue kaneBlockingQueue = new BlockingQueue(6);
        new Thread() {
            @Override
            public void run() {
                for (int i = 0; i < 10; i++) {
                    kaneBlockingQueue.put("x" + i);
                }
            }
        }.start();
        Thread.sleep(1000L);
        System.out.println("開(kāi)始取元素");
        for (int i = 0; i < 8; i++) {
            kaneBlockingQueue.take();
            Thread.sleep(2000);
        }
    }
}
class BlockingQueue {
    List<Object> list = new ArrayList<>();
    private Lock lock = new ReentrantLock();
    private Condition putCondition = lock.newCondition(); //condition可以有多個(gè)，針對(duì)不同的操作放入不同condition,相當(dāng)于等待隊(duì)列
    private Condition takeCondition = lock.newCondition();
    private int length;
    public BlockingQueue(int length) {
        this.length = length;
    }
    public void put(Object obj) {
        lock.lock(); //思考一個(gè)讀一個(gè)寫，為什么要加鎖呢？
        try {
            while (true) {
                if (list.size() < length) { //我們集合的長(zhǎng)度不能超過(guò)規(guī)定的長(zhǎng)度，才能向里面放東西
                    list.add(obj);
                    System.out.println("隊(duì)列中放入元素:" + obj);
                    takeCondition.signal();
                    return;
                } else { //如果放不進(jìn)去，就該阻塞. --利用condition實(shí)現(xiàn)
                    putCondition.await();//掛起
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
    public Object take() {
        lock.lock();
        try {
            while (true) {
                if (list.size() > 0) {
                    Object obj = list.remove(0);
                    System.out.println("隊(duì)列中取得元素:" + obj);
                    putCondition.signal();
                    return obj;
                } else {
                    takeCondition.await();
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
            return null;
        }
    }
}

隊(duì)列中放入元素:x0
隊(duì)列中放入元素:x1
隊(duì)列中放入元素:x2
隊(duì)列中放入元素:x3
隊(duì)列中放入元素:x4
隊(duì)列中放入元素:x5
開(kāi)始取元素
隊(duì)列中取得元素:x0
隊(duì)列中放入元素:x6
隊(duì)列中取得元素:x1
隊(duì)列中放入元素:x7
隊(duì)列中取得元素:x2
隊(duì)列中放入元素:x8
隊(duì)列中取得元素:x3
隊(duì)列中放入元素:x9
隊(duì)列中取得元素:x4
隊(duì)列中取得元素:x5
隊(duì)列中取得元素:x6
隊(duì)列中取得元素:x7

Process finished with exit code 0

可重入鎖 ReentrantLock

一般來(lái)說(shuō)，如果可重入鎖的加鎖次數(shù)是n，那么解鎖次數(shù)也得是n才能完全釋放鎖，否則，如果小于n 則無(wú)法正常釋放鎖，此時(shí)如果有別的線程要加鎖，則無(wú)法獲取到鎖而被阻塞；如果大于n，則會(huì)觸發(fā) IllegalMonitorStateException 異常， ReentrantLock 默認(rèn)是使用非公平鎖，如果要使用公平鎖，可以使用 new ReentrantLock(true) 來(lái)創(chuàng)建。

小于n的情況

package lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Reentrant_Demo {
    static Lock lock = new ReentrantLock();
    public static void main(String[] args) {
        lock.lock();
        System.out.println(Thread.currentThread().getName() + "獲得第1次鎖");
        lock.lock();
        System.out.println(Thread.currentThread().getName() + "獲得第2次鎖");
        lock.unlock();
        new Thread() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + "開(kāi)始去釋放鎖");
                lock.lock();
                System.out.println("獲得鎖成功~~~");
                lock.unlock();
            }
        }.start();
    }
}

main獲得第1次鎖
main獲得第2次鎖
Thread-0開(kāi)始去釋放鎖
// 子線程獲取鎖失敗導(dǎo)致阻塞了

大于n的情況修改成3次unlock

        // 修改成3次unlock
        lock.unlock();
        lock.unlock();
        lock.unlock();

main獲得第1次鎖
main獲得第2次鎖
Exception in thread "main" java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:151)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1261)
    at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:457)
    at lock.Reentrant_Demo.main(Reentrant_Demo.java:18)

簡(jiǎn)單說(shuō)明圖

實(shí)現(xiàn)一個(gè)ReenrantLock的demo版本 - 一個(gè)現(xiàn)實(shí)思想的簡(jiǎn)單版本

package lock;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.LockSupport;
public class ReentrantLock_Demo implements Lock {
    //記錄鎖的擁有者
    AtomicReference<Thread> owner = new AtomicReference<>();
    //記錄重入次數(shù)的count
    AtomicInteger count = new AtomicInteger(0);
    //等待隊(duì)列
    private LinkedBlockingQueue<Thread> waiters = new LinkedBlockingQueue();
    @Override
    public boolean tryLock() {
        //判斷count值是否為0，如果count不等于0，說(shuō)明鎖被占用
        int ct = count.get();
        //判斷鎖是不是自己占用的，做重入
        if (ct != 0) {
            if (Thread.currentThread() == owner.get()) {
                count.set(ct + 1);
                return true;
            }
        } else { //若count為0 ，表示當(dāng)前鎖未被占用，通過(guò)CAS操作
            if (count.compareAndSet(ct, ct + 1)) {
                owner.set(Thread.currentThread());          //如果不是自己，進(jìn)入隊(duì)列
                return true;
            }
        }
        return false;
    }
    @Override
    public void lock() {
        if (!tryLock()) {
            //加入等待隊(duì)列
            waiters.offer(Thread.currentThread());
            while (true) {
                //若線程是隊(duì)列頭部，先判斷一次，現(xiàn)在能不能去搶，然后再去加鎖
                Thread head = waiters.peek();
                if (head == Thread.currentThread()) {
                    if (!tryLock()) {
                        LockSupport.park();
                    } else {
                        waiters.poll();
                        return;
                    }
                } else {
                    LockSupport.park();
                }
            }
        }
    }
    public boolean tryUnlock() {
        if (owner.get() != Thread.currentThread()) {
            throw new IllegalMonitorStateException();
        } else {
            int ct = count.get();
            int nextc = ct - 1;
            count.set(nextc);
            if (nextc == 0) { //可重入鎖被加鎖多次，一旦為0 就釋放鎖，如果不是0，還得繼續(xù)釋放
                owner.compareAndSet(Thread.currentThread(), null);
                return true;
            } else {
                return false;
            }
        }
    }
    @Override
    public void unlock() {
        if (tryUnlock()) {
            Thread head = waiters.peek();
            if (head != null) {
                LockSupport.unpark(head);
            }
        }
    }
    /**
     * 暫時(shí)忽略
     *
     * @throws InterruptedException
     */
    @Override
    public void lockInterruptibly() throws InterruptedException {
    }
    /**
     * 暫時(shí)忽略
     *
     * @throws InterruptedException
     */
    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return false;
    }
    /**
     * 暫時(shí)忽略
     *
     * @throws InterruptedException
     */
    @Override
    public Condition newCondition() {
        return null;
    }
}

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